"Like in similar products, Google maps
ETAs are based on a variety of things,
depending on the data available in a particular area. These
things range from official speed limits and recommended speeds,
likely speeds derived from road types, historical average speed
data over certain time periods (sometimes just averages,
sometimes at particular times of day), actual travel times from
previous users, and real-time traffic information. They mix data
from whichever sources they have, and come up with the best
prediction they can make.

Most companies who do live traffic compare their
predictions against actual time in traffic to tune their
algorithms and data sources. The likely result of this is that
the companies who have access to the best usage data (ie those
who are best able to compare their predictions against reality,
which means those who have the most usage) are likely to end up
with the best predictions in the medium to long term."

In short, there's a ton of data Google is calculating just to
tell you that your destination may happen to be 10 minutes away.
The company also revealed more on how crowdsourced traffic
data helps make Maps even more accurate in 2009.

"When we combine your speed with the speed of other phones
on the road, across thousands of phones moving around a city at
any given time, we can get a pretty good picture of live traffic
conditions," wrote Dave Barth, product manager for Google
Maps.

Of course, no matter how much data is involved, the time you get
will likely never be perfect. As Russell writes, calculating ETAs
"is a future-prediction problem, and traffic, while it
follows certain patterns, is inherently unpredictable."

Google may know a ton of information, but it doesn't know
about the car crash that may have just happened or the school bus
making multiple stops in front of you.